The solid-state transformer (SST) was regarded as the ideal power router in the energy Internet. However, the partial discharge (PD) in tightly packed SST is one of the main causes of interturn multilayer insulation premature failure. The effect of high-frequency (HF) electrical-thermal coupling stress on SST multilayer insulation's PD characteristics and failure mechanism is not clarified, which is investigated in this article. An HF PD measurement system is established, and a comparative study of the PD characteristics of multilayer polyimide (PI) insulation at different test frequencies and temperatures is explored. The experimental results show that when the frequency exceeds 10 kHz, the PD exhibits a rabbit-ear-like discharge pattern. However, the higher temperature will lead to lower PD inception voltage with large PD magnitudes, which may accelerate the degradation of interturn insulation. According to the phase-resolved PD, PD magnitude, PD number, and discharge phenomenon of the PI layer, the failure process of the multilayer PI system is divided into three stages: electric tree development, electric tree breakdown, and point prebreakdown. In addition, we propose the PD development model of multilayer PI insulation. The research results can provide a theoretical reference for elucidating the PD mechanism under HF stresses.

Due to the coefficients of thermal expansion (CTE) mismatch between various materials, the bond wire of insulated gate bipolar transistor (IGBT) module is prone to heel-cracking or lift-off failure under power cycling, which will affect the performance of the module. In this article, an electro-thermal–mechanical multiphysics coupling model was constructed for IGBT module, and the accuracy of the model was validated by steady-state conduction experiment. Based on the simulation model, the characteristics of IGBT module were further simulated under the normal state conduction operation, and the results showed that the stress of the middle bond position on a single bond wire is higher. Then, the research on the influence of heel-cracking and lift-off failure of the bond wire was carried out. The simulation results showed that the heel-cracking failure of the bond wire will cause obvious effect, in which the increase stress can be as high as 77.4 MPa. When the bond wires are detached one by one until five bond wires were lifted off, the temperature in the remaining bond wires increases by 48.7 $^{\circ}$ C, which has a significant impact on the module. The research in this article can provide the basis for improving the reliability of IGBT module and the optimization of module package design.

High-frequency power transformers are conducive to the reliable grid connection of distributed energy sources. Polyimide is often used for the coating insulation of high-frequency power transformers. However, creeping discharge will cause insulation failure, therefore, it is necessary to use disiloxane for the purpose of modifying the molecular structure of polyimide. This paper not only introduces 1,3-bis(3-aminopropyl)-1,1,3,3-tetramethyldisiloxane (GAPD) with a molar content of 1%, 2%, and 5% to polyimide, but also tests both the physical and chemical properties of the modified film and the high frequency creeping dielectric strength. The results show that after adding GAPD, the overall functional groups of the material do not change, at the same time the transfer complexation of intermolecular charge and the absorption of ultraviolet light increase. There is no phase separation of the material and the structure is more regular and ordered, moreover the crystallinity increases. The overall dielectric constant and the dielectric loss tangent value show different trends, which means that the former value increases, while the latter value decreases. In addition, the resistivity of the surface and the volume increase, which is the same as the glass transition temperature. The mechanical properties are excellent, and the strength of bulk breakdown is mounting. The insulation strength of the high frequency creeping surface has been improved, which will increase with larger contents of GAPD. Among them, the relative change of the creeping flashover voltage is not obvious, and the creeping discharge life of G5 is 4.77 times that of G0. Further analysis shows that the silicon-oxygen chain links of the modified film forms a uniformly dispersed Si-O-Si network in the matrix through chemical bonds and charge transfer complexation. Once the outer matrix is destroyed, it will produce dispersed flocculent inorganic particles which have the role of protecting the inner material and improving the performance of the material. Combined with the ultraviolet light energy absorption, the increase of deep traps, the reduction of dielectric loss, and the improvement of thermodynamic performance, can better improve the high-frequency creeping insulation strength of polyimide film and its potential application value.

The air cavities at the interface are an important cause of partial discharge (PD) in SST composite insulation. Therefore, it is of great practical significance to study the partial discharge and breakdown characteristics of composite insulation air cavities under high frequency electric stress. In this paper, the interface breakdown strength and partial discharge characteristics under different interface contact morphologies were studied by building a high frequency discharge test system. Experimental results of interface discharge show that breakdown voltage is significantly affected by surface roughness and contact pressure. When the surface roughness increases from 1.23 μm to 4.39 μm, the interface breakdown voltage of the air gap defect at # 2000 mesh decreases from 19.34 kV to 12.91 kV. Under different contact pressures, the variation of breakdown voltage of air cavities with surface roughness is basically consistent. The results of PD characteristics show that, increasing the surface roughness will reduce the partial discharge inception voltage of the air cavities, and the partial discharge amplitude and discharge times will increase with the surface roughness. Combining the effects of secondary electron emission yields (SEEY), resistivity and surface roughness on charge dissipation process, the formation mechanism of interface discharge induced by air cavities under high frequency stress is revealed. This work can provide a theoretical reference for the study of the effect of air cavities on composite insulation characteristics under high frequency stress.

Inter-turn insulation made of polyimide (PI) is considered to be the weakest part of the solid-state transformers (SST) composites insulation system due to the existence of an interface between the cast epoxy resin (EP) and itself. Ion-exchange strategy can be used to regulate the dielectric and electrical properties of PI insulating dielectric to ensure the stability of the composite insulation system. Therefore, this paper intends to investigate the effects of the ion-exchange strategy on the space charge accumulation and interface insulation performance between PI and EP composites insulation. Our experimental results demonstrate that the wide-bandgap alumina (Al ₂ O ₃ ) nanoparticles can effectively improve the dielectric compatibility of PI and EP and significantly suppress the space charge accumulation on the PI/EP interface. The novel PI/EP composite structures insulation with breakdown strength about 39.5% higher than that of traditional PI/EP is obtained. The improvement mechanism is that the Al ₂ O ₃ layer introduces a large number of deep traps on the surface of the PI film as well as reduces its dielectric compatibility with the EP, suppresses charge accumulation at the PI/EP composites insulation interface, and thus enhances the high-frequency insulation performance. In this paper, the proposed surface engineering strategy is convenient and scalable, providing new insights into the regulation of composite insulation interfacial compatibility in high-voltage power equipment.